The identification of the Bcl2 as well as lAP family members has suggested that excessive inhibition of apoptosis may constitute a common feature of all known human cancers -the ability to influence their onset, progression and outcome. The crystal structure of BCl-xL (a member of Bcl2 family) has contributed to a better understanding of its functional domains including the discovery of an unstructured "loop region" (LR) near the N-terminus exposed to the cytoplasm. The regulation of phosphorylation of Bcl-xL on Serine 62 residue in this "LR" by tubulin-binding anticancer drugs at mitosis has been defined. Importantly, the expression of a phosphorylation null (Ser 62 Ala) mutant Bcl-xL enhanced the growth of an established prostate tumor cell line in vitro by reducing the apoptotic threshold. Further in vitro studies indicate a novel downstream interaction of phosphorylated Bcl-xL with Pin1, a member of peptidyl prolyl isomerase family. To comprehend the significance of their association, elaborative studies will be undertaken in cancer cells, where the Pin1 gene will be post-transcriptionally silenced by a siRNA approach. To the other end, the importance of Bcl-xL expression during B cell development is demonstrated by the expansion of the pro-pre-B cell compartment. Placing the phosphomimetic and phosphorylation-defective mutant of Bcl-xL transgenically into the germ line of mice will provide a prospective opportunity to observe the effects of phosphorylation during the development of lymphocytes. Besides, the effect of microtubule-damaging drugs, such as Taxol or 2-Methoxyestradiol (2-ME), will be examined in tumor xenografts developed in athymic mice by implanting prostate cancer cells overexpressing phosphomimetic and phosphorylation-defective mutant of Bcl-xL. The following specific aims are proposed. Aim 1: To determine the outcome of the association between phospho-BCl-xL and Pin1. Aim 2: In vivo function of Bcl2 phosphorylation in regulating B lymphocyte development using a transgenic mouse model. Aim 3: To study the effect of Taxol or 2-ME on the tumors developed by prostate cancer cells overexpressing wild, phosphomimetic and phosphorylation-defective mutant of Bcl-xL in athymic (nude) mice. The understanding yielded from the proposed studies might be helpful to target a prospective therapy for a subpopulation of cancer cells in which Bcl-xL overexpression is a key prognostic factor.